rs1421085 is a SNP located in the first intron of the FTO (fat mass and obesity associated) gene on chromosome 16. This SNP showed the most association with obesity in the original work by Dina et al [PMID 17496892].

[PMID 26287746] The rs1421085 T-to- C single-nucleotide alteration underlies the association between FTO and obesity by disrupting ARID5B-mediated repression of IRX3 and IRX5. This disruption leads to a developmental shift from browning to whitening programs and loss of mitochondrial thermogenesis

Within the FTO gene, many SNPs appear to be co-inherited. The SNP showing the strongest association with body weight (i.e. body mass index, BMI) is not rs1421085, although this SNP is one of co-inherited SNPs in the FTO gene region. For more information, refer to the FTO gene or the most studied of FTO SNPs, rs9930506. [PMID 17658951]

However, in one study of 583 extremely obese women with several FTO SNPs, the strongest association was indeed with rs1421085 (p=3.04x10-10, OR = 1.75, CI: 1.47-2.08).[PMID 18218107]

Note: The three FTO SNPs, rs1421085, rs17817449, and rs9939609, are in strong linkage disequilibrium (pairwise r2>0.97), and there are two primary haplotypes, C-G-A (42.0 %) and T-T-T (55.5 %).

[PMID 23341774] A Systematic Mapping Approach of 16q12.2/FTO and BMI in More Than 20,000 African Americans Narrows in on the Underlying Functional Variation: Results from the Population Architecture using Genomics and Epidemiology (PAGE) Study